in physiology, the process by which a blood clot is formed. The formation of a clot prevents blood loss from a ruptured blood vessel. Under abnormal circumstances, clots can also form in a vessel that has not been breached; such clots can result in the occlusion of the vessel.

Clotting is a sequential process that involves the interaction of numerous blood components called coagulation factors. Among the better-known factors are fibrinogen (see fibrin) and prothrombin (q.v.) (factors I and II) and antihemophilic globulin (factor VIII). There are 13 principal coagulation factors in all, and each of these has been assigned a Roman numeral up to XIII.

The first step in coagulation is the formation of prothrombin activator. This substance forms in response to compounds released by the damaged vessel wall. Prothrombin activator also forms in response to changes in the blood itself; these changes result from the blood contacting the collagen fibres of the ruptured vessel.

Once formed, prothrombin activator initiates the conversion of prothrombin to thrombin. Thrombin, in turn, catalyzes the conversion of fibrinogen (fibrin)—a soluble plasma protein—into long, sticky threads of insoluble fibrin. The fibrin threads form a mesh that traps platelets, blood cells, and plasma. Within minutes, the fibrin meshwork begins to contract, squeezing out its fluid contents. This process, called clot retraction, is the final step in coagulation. It yields a resilient, insoluble clot that can withstand the friction of blood flow.